The present invention relates to an image forming apparatus of an electrophotographic system which forms images by transferring onto a transfer material toner images formed on an image carrier and by fixing them on the transfer material in a copying machine, a printer or a facsimile machine, and in particular, to an image forming apparatus capable of forming images on any of the obverse side and the reverse side of a transfer material by transferring toner images formed on an image carrier onto the obverse side of the transfer material, or by transferring the toner images onto the reverse side of the transfer material after transferring them temporarily onto a toner image receptor.
In the case of two-sided copy, there has been employed a method wherein an image for one side of a transfer material formed on an image carrier is transferred and fixed on the transfer material, then the transfer material is stored temporarily in an intermediate tray, and then is fed out of the intermediate tray in synchronization with an image formed again on the image carrier so that the image may be transferred and fixed on the other side of the transfer material.
This two-sided copy apparatus has been less reliable in conveyance of a transfer material and has caused a paper jam because the transfer material needs to be fed into and fed out of an intermediate tray and it needs to pass through a fixing unit twice. In contrast to the foregoing, TOKKOSHOs 49-37538 and 54-28740 and TOKKAIHEIs 1-44457 and 4-214576 disclose a technology wherein fixing is conducted only once after forming toner images on both sides of a transfer material, and in particular, TOKKAIHEIs 1-44457 and 4-214576 disclose a method wherein a plurality of image forming means each being composed of an image carrier, a charging means, an image writing means and a developing means are arranged in parallel on a toner image receptor to form a two-sided copy of color images.
However, in two-sided color image forming disclosed by TOKKAIHEIs 1-44457 and 4-214576 mentioned above, image deteriorations such as color doubling or scattering and rubbing of toner are easily caused because color toner images each being different in color are superposed one by one on a toner image receptor, although conveyance of a transfer material is improved.
In contrast to this, inventors of the invention are studying, as they propose in TOKKAIHEIs 8-70322 and 8-70323, an image forming apparatus wherein a toner image formed on the first image carrying means (photoreceptor drum) is transferred onto the second image carrying means (toner image receptor) temporarily, then a toner image is formed again on the photoreceptor drum, and the toner image on the toner image receptor is transferred as a reverse side image onto the reverse side of the transfer material conveyed on the toner receptor, while the toner image formed again on the photoreceptor is transferred as an obverse side image onto the obverse side of the transfer material, and then the transfer material is separated from the toner image receptor and toner images on the both sides of the transfer material are fixed by a fixing means to form two-sided images. This method, however, has a problem that a crease is caused on a transfer material which enters a nip portion of a fixing means when the transfer material is conveyed to the fixing means, and thereby the toner images on both sides of the transfer material can not be fixed satisfactorily. There is further caused a problem that the transfer material is lifted to touch the second transfer means and a transfer material separating means when it is separated from the second image carrying means in a belt shape and is conveyed to the fixing means, resulting in disturbed toner image on the obverse image.
The first object of the invention is to solve the problems mentioned above and to provide an image forming apparatus wherein a transfer material is caused to enter a fixing means without being creased and is less brought into contact with the second transfer means or with a transfer material separating means when the transfer material is conveyed from the second belt-shaped transfer means to a fixing means.
Another problem to be solved by the invention will be explained as follows. Namely, the second image carrier (toner image receptor) is provided to be adjacent to or to be in contact with a photoreceptor drum, a driving roller is provided at the downstream side and a driven roller is provided at the upstream side both from the position where the toner image receptor is provided to be close to or to be in contact with the photoreceptor drum in the rotary direction of the toner image receptor, respectively to be inscribed, and a transfer material charging means (paper charging unit) which makes a transfer material to be attracted to the toner image receptor and the second image carrying means cleaning blade (toner image receptor cleaning blade) of the second image carrying means cleaning means which comes in contact with the toner image receptor to remove the toner remaining thereon after transferring are provided so that they face the driven roller through the toner image receptor. However, the shock caused when the toner image receptor cleaning blade comes in contact with or leaves the toner image receptor makes the toner image receptor to have uneven rotation which causes a blurred image on a toner image of the obverse image or a reverse image to be transferred onto a transfer material that is conveyed by the toner image receptor, which is a problem.
There is further caused a problem that a paper charging unit arranged in the vicinity of the toner image receptor cleaning means is contaminated by toner spattered from the toner image receptor cleaning means.
The second object of the invention is to solve the problems mentioned above and to provide an image forming apparatus wherein uneven rotation of the second image carrying means caused by the shock generated when the second image carrying means cleaning blade comes in contact with or leaves the second image carrying means is prevented, and contamination of a transfer material charging means caused by toner spattered from the second image carrying means cleaning means is prevented.